This invention relates to a method and an apparatus for producing a photomask blank (hereinafter may simply be referred to as a mask blank) used to obtain a photomask (including a reticle) and including a transparent substrate on which a phase shift film, an opaque film, or a stack of the phase shift film and the opaque film is formed, and a resist applied thereon. This invention particularly relates to a method and an apparatus for producing a photomask blank from which an unnecessary film, i.e., an unnecessary part of a resist film is removed, which is formed in a peripheral portion or a portion of the edge of a substrate as a result of resist application. This invention also relates to an apparatus for removing the unnecessary portion of a film.
In the lithography technology as a method of producing a semiconductor device (such as LSI), a photomask (including a reticle) is used for transfer of circuit patterns. In order to produce the mask, a blank is used as a material. Referring to FIG. 1, the blank depicted at 5 in the figure is obtained in the following manner. On a transparent substrate 1 made of synthetic quartz or the like, an opaque film 2 made of chromium as a main component is formed on the transparent substrate 1. Then, an antireflection film 3 made of chromium oxide as a main component is continuously or discontinuously formed on the opaque film 2 by sputtering or the like. Thereafter, a resist is applied onto the antireflection film 3 by spin coating or the like, followed by heat treatment (post-application bake (pre-bake or soft bake)) for drying up to thereby form a resist film 4.
When the resist is applied as mentioned above, a relatively thick resist part 6 is formed in a portion of the edge of a substrate (i.e., a zone along four sides of a principal surface of the substrate 1 and an end face (side surface) of the substrate 1), as shown in FIG. 2 which is an enlarged view of an encircled part A in FIG. 1.
When the blank 5 is put into or taken out from a container, the resist part 6 in the portion of the edge of the substrate 1 is easily flaking (peeled off and released) as a result of contact and friction with the container and re-attached as a dust to the mask blank 5 itself or various apparatuses in a mask making process. This results in occurrence of a defect in the mask (including the reticle), which is a product obtained from the blank as a material, or in reduction of a production yield of the mask.
In order to solve the above-mentioned problem, it is required to remove an unnecessary part of the resist, i.e., an unnecessary resist film, which is formed in the portion of the edge of the substrate. As a technique of removing the unnecessary resist film, a method has been widely in use, which comprises the steps of selectively dissolving and removing the unnecessary resist film by the use of an organic solvent or the like after the resist is applied or after the post-application bake is carried out (for example, see Japanese Examined Patent Publication No. S57-13863 (JP-B)). The above-mentioned method will hereinafter referred to as a first existing method.
In the first existing method mentioned above, the unnecessary resist film formed in the portion of the edge of the substrate (a substrate end face 7, a chamfered face 8, a principal surface edge 9, and a resist removal area (a naked area of the opaque film) 10 on a principal surface) is removed, as illustrated in FIG. 3. However, a removal end (endmost portion) 11 left at a boundary between a resist film 12 and the resist removal area 10 has a remarkable edge roughness (irregularity or unevenness). In addition, as shown in FIG. 3A which is a sectional view taken along a line X1-X2 in FIG. 3, a remarkable protrusion 13 is formed at the removal end 11. Thus, the quality of resist removal is inferior.
In order to avoid the disadvantage in the first existing method using the organic solvent, another technique has widely been in use, which is for removing the unnecessary resist film formed in the portion of the edge of the substrate (although this technique is exclusively applicable to a positive resist). In case of a silicon wafer, the resist is applied to the wafer and the post-application bake is carried out. Thereafter, the unnecessary resist film formed in the portion of the edge of the substrate is selectively exposed by the use of a light source for emitting light having a wavelength within a range to which the resist is sensitive (generally, in an ultraviolet (UV) range or a deep-ultraviolet (DUV) range). Then, a main pattern is exposed. In a developing step subsequently carried out, formation of the main pattern and development/removal of the unnecessary resist film are simultaneously carried out. This technique will hereinafter be referred to as a second existing method.
The unnecessary resist film in the portion of the edge of the substrate is selectively exposed as an exposed part after the resist is applied and the post-application bake is carried out. Alternatively, the exposed part is selectively developed and removed immediately after the exposure prior to the main pattern exposure and development. For example, such periphery exposure technique is disclosed in Japanese Unexamined Patent Publication No. S63-160332. This technique will hereinafter be referred to as a third existing method.
In particular, in a production process of the blank and in a production process of the mask, the removal of the unnecessary resist film in the portion of the edge of the substrate must be completed at a stage when the blank is finished (i.e., at a stage when the application of the resist and the post-application bake are finished). Specifically, before the blank is inserted and packed into a storage container for the purpose of shipment and storage, the unnecessary resist film in the portion of the edge of the substrate must preliminarily be removed. Thereafter, the blank in the storage container is sent to the mask making process.
By the use of the periphery exposure technique in the third existing method mentioned above, it is possible to produce the blank that has the unnecessary resist film removed before being sent to the mask making process. Specifically, in the production process of the blank, the unnecessary resist film in the portion of the edge of the substrate is selectively exposed as an exposed part after the resist is applied and the post-application bake is carried out. Then, the exposed part is selectively developed and removed.
However, the periphery exposure technique in the third existing method is disadvantageous in the following respects because the periphery exposure is carried out after the resist is applied and the post-application bake is carried out.
At first, in case of a chemically-amplified (CA) resist recently and widely spread, after the resist is applied and the post-application bake is carried out, the blank is subjected to post-exposure bake (PEB) after exposure. Then, the exposed part begins to exhibit a sufficient solubility in a developer chemistry. Therefore, it is required to include the step of selectively exposing the unnecessary resist film, the step of performing the post-exposure bake (PEB), and the step of developing and removing the unnecessary resist film particularly in order to remove an unnecessary chemically-amplified resist film in the portion of the blank substrate edge by the use of selective exposure after resist is applied and post-application bake is carried out. In addition, apparatuses or mechanisms for carrying out these steps are required also. This results in an increase in scale of the process and the apparatus arrangement involved in resist application and in an increase in cost. Hereinafter, the above-mentioned technique will be referred to as a fourth existing method.
Second, in order to remove the unnecessary chemically-amplified resist film in the portion of the edge of the substrate by the use of selective exposure, the step of selectively exposing the unnecessary resist film must be followed by the post-exposure bake (PEB), as described above. However, it is practically impossible to carry out the post-exposure bake (PEB) selectively and exclusively upon the unnecessary portion of the resist film. As far as a typical baking apparatus (for example, a hot plate) is used, a patterning area of the principal surface of the substrate where the main pattern will later be formed is inevitably baked. Specifically, as compared with a blank obtained without removing the unnecessary portion of the resist film, another or one more excess baking step is additionally carried out. As a consequence, the sensitivity of the resist film in the patterning area of the principal surface of the substrate and the quality (for example, contrast) of the pattern to be formed are inevitably varied and deteriorated.
In the meanwhile, an electron beam (EB) lithography/exposure resist is mainly used in production of a high-accuracy mask. The chemically-amplified resist for EB lithography/exposure is generally sensitive to a light of the deep UV (ultraviolet) range. Therefore, it is possible to selectively expose, develop, and remove the unnecessary portion of the chemically-amplified resist film selectively by the use of a light source in the deep UV (ultraviolet) range in accordance with the fourth existing method described above.
On the other hand, in a high-molecular weight polymer-type EB lithography/exposure resist having little sensitivity to the deep UV (ultraviolet) range, it is not practical to expose the unnecessary portion of the resist film selectively by the use of an exposure light source in the deep UV (ultraviolet) range. For example, a high-molecular weight polymer-type resist made of a copolymer of alpha methyl styrene and alpha chloroacrylate is applied and subjected to post-application bake, and followed by periphery exposure by the use of a deep UV (ultraviolet) lamp under an exposure condition practical and particular for the chemically-amplified resist. In this event, there arises a third problem that the resist is not dissolved at all in a developer (organic solvent) chemistry.
In view of the above-mentioned background, this invention is made. This invention has following structures.
Structure 1
A method of producing a photomask blank, comprising a thin film forming step of forming, on a rectangular (including square) substrate, a thin film for causing an optical change in exposure light, a resist application step of applying a positive resist on the thin film, a baking step of heat treating the resist applied on the thin film, and a removing step of removing a resist film formed in a portion of an edge of the substrate, wherein the removing step is carried out by exposing the resist film in the portion of the edge of the substrate after the resist application step and before the baking step so that, upon development subsequently carried out in the removing step, a difference in solubility or dissolving speed in a developer is obtained between an exposed area and an unexposed area and by selectively supplying the developer to the exposed area.
Structure 2
A method of producing a photomask blank as described in structure 1, wherein the removing step by the development is carried out before or after the baking step.
Structure 3
A method of producing a photomask blank as described in structure 1 or 2, wherein the photomask blank is covered at least in its peripheral portion by a cover member with a predetermined distance left between a principal surface of the photomask blank and the cover member, the predetermined distance being selected so that the developer is filled exclusively in a gap defined by the predetermined distance for capillary action (phenomenon) when the developer is supplied to the gap, the developer being supplied to the gap defining a developer supply area on the principal surface of the photomask blank so that the resist film formed in the portion of the edge of the substrate is removed.
Structure 4
A method of producing a photomask blank, comprising a thin film forming step of forming, on a rectangular (including square) substrate, a thin film for causing an optical change in exposure light, a resist application step of applying a positive resist on the thin film, and a baking step of heat treating the resist applied on the thin film, wherein the resist film in the portion of the edge of the substrate is exposed after the resist application step and before the baking step so that, in a developing step during a photomask production process, the resist film in the portion of the edge of the substrate exposed as mentioned above is removed simultaneously with formation of a mask pattern.
Structure 5
A method of producing a photomask blank as described in any one of structures 1 through 4, wherein the resist application step comprises a leveling (uniforming) step of dispensing an amount of a resist solution onto the rectangular substrate and rotating the substrate at a predetermined principal rotation speed for a predetermined principal rotation time mainly to achieve a uniform thickness of the resist, and a drying step of rotating, after the leveling step, the substrate at a predetermined dry rotation speed lower than the principal rotation speed for a predetermined dry rotation time mainly to dry the resist having the uniform thickness.
Structure 6
A method of producing a photomask blank as described in any one of structures 1 through 5, wherein the resist is a chemically-amplified resist.
Structure 7
An unnecessary film removing apparatus for removing, from a positive resist film formed on a surface of a photomask blank, an unnecessary part formed in a portion of an edge of the substrate as an unnecessary film, the apparatus comprising:
exposure means for exposing the unnecessary part of the positive resist film formed in the portion of the edge of the substrate;
baking means for heat treating the positive resist film; and
removing means for selectively supplying a developer to a resist-removing region of the substrate where the positive resist film is to be removed, and removing the unnecessary film by development.
Structure 8
An unnecessary film removing apparatus as described in structure 7, wherein the removing means comprises holding means for holding the photomask blank horizontally so that the photomask blank is rotatable around a vertical axis, a cover member at least covering the portion of the edge of the photomask blank and provided with a distance adjusting member for adjusting a distance between a principal surface of the photomask blank and the cover member so that a developer is filled exclusively in a gap defined by the distance when the developer is supplied to the gap, and developer supply means for supplying the developer to the gap.
Structure 9
An unnecessary film removing apparatus as described in structure 7 or 8, wherein the exposure means comprises holding means for holding the photomask blank horizontally so that the photomask blank is scanning linearly along an axis, exposure means for irradiating the portion of the edge of the photomask blank with light, and driving means for driving one or both of the exposure means and the holding means so that the exposure means carries out scanning along the portion of the edge of the photomask blank.
Structure 10
An unnecessary film removing apparatus as described in structure 9, wherein the driving means comprises scanning speed adjusting means for changing a scanning speed so that an accumulated or integrated exposure energy of light emitted by the exposure means is changed in dependence upon various thickness of the unnecessary film formed in the portion of the edge of the photomask blank.
Structure 11
An unnecessary film removing apparatus as described in any one of structures 7 through 10, wherein the resist is a chemically-amplified resist.
Structure 12
A photomask blank producing apparatus comprising a resist application apparatus for applying a positive resist on a surface of a photomask blank including a rectangular substrate and a thin film formed on the substrate for causing an optical change in exposure light, and an unnecessary film removing apparatus described in any one of structures 7 through 11, the unnecessary film removing apparatus being arranged so that the exposure means, the removing means, and the baking means are disposed in this order along a conveying line of the photomask blank.
Structure 13
A photomask blank producing apparatus comprising a resist application apparatus for applying a positive resist on a surface of a photomask blank including a rectangular substrate and a thin film formed on the substrate for causing an optical change in exposure light, and an unnecessary film removing apparatus described in any one of structures 7 through 11, the unnecessary film removing apparatus being arranged so that the exposure means, the baking means, and the removing means are disposed in this order from an upstream side towards a downstream side along a conveying line of the photomask blank.